PROJECT SUMMARY Recent advances in microscopes, detectors and computational image processing have revolutionized structural analysis of biological macromolecules by cryo-electron microscopy (cryo-EM). These advances are allowing researchers to generate atomic models of biomedically relevant molecules that are refractory to other structural techniques. The University of California at Santa Cruz hosts several NIH-funded structural biology researchers who are advancing studies of complex cellular machineries including the ribosome, spliceosome, telomerase, cell-cycle regulators, circadian clock, childhood viruses, immune response regulators and amyloidogenic polypeptides. Cryo-EM holds huge promise to advance our research programs, but we need to update and expand our local capabilities to bring new structural studies to fruition. We request funding for a 200 kV transmission electron microscope with a direct electron detector, and bundled with an automated plunge freezer. The instrument will replace an aging 120 kV instrument with limited cryo-EM capabilities and an obsolete operating system. A 200 kV instrument designed for cryo-EM data collection will allow a talented group of structural biologists to realize the promise of cryo-EM to determine the structures of larger and more challenging complexes of proteins and nucleic acids and reveal the molecular basis of a variety of human diseases. The instrumentation will also complement our membership in a consortium of our sister institutions in the San Francisco Bay Area, which gives us access to the most advanced cryo-EM instrumentation to push the structures that we first solve at UC Santa Cruz to the highest resolution possible.